This invention relates to an image processing method for storing hierarchically coded image data, decoding the stored data and then outputting the same, as well as an apparatus for practicing this method.
Methods which can be utilized for coding binary image data in the prior art include MH, MR and MMR methods. According to these coding methods, an image is coded sequentially from the top down. Consequently, in a case where an original image having a high resolution of 400 dpi, for example, is displayed on a computer display unit (a low-resolution CRT) or the like, the arrangement is such that the coded data is decompressed and converted into the original image data, after which the resulting image is displayed.
A "Progressive Bi-level Image Compression Method-JBIG (Joint Bi-level Image Group) Algorithm" is described in THE JOURNAL OF THE INSTITUTE OF IMAGE ELECTRONICS OF JAPAN (1991, Vol. 20, No. 1). This coding method is a method of progressive bi-level image compression. This is a method of variable-length coding, in which the coded data is managed as one file.
It is possible also to adopt an arrangement in which the original image data is held directly without performing the coding of the binary image data.
However, if the original image has a resolution of 400 dpi in size A4, for example, the total number of pixels is 4752.times.3360 pixels in the above-described example of the prior art. Since the resolution of a CRT for displaying images is on the order of 1024.times.1024, the following problems arise:
(1) In a case where detailed data in a certain rectangular area within a reduced image displayed on a CRT is to be displayed in enlarged form or the entire image is desired to be enlarged, enlarging the reduced image data results in enlargement of the image size, but there is no increase in resolution and special-purpose hardware for enlargement is required. Further, in a case where reduction is carried out again from the original image, performing this based upon coded data requires that execution be carried out again upon changing the reduction ratio, as mentioned in (1) above. This takes time. Furthermore, in a case where an original image whose coded data has been decompressed is stored in a memory in advance and the original image is reduced directly without performing the coding, a buffer memory [having a capacity of (4752.times.3360)/8 bytes if the original image is composed of 4752.times.3360 pixels] is required in order to store the original image in its entirety.
(2) In case of the JBIG method, hierarchical coding is performed, in which coding is carried out for every resolution.. If the coded data (hereinafter referred to as a "bit stream") is one file, there is no tag information such as the bit length of every layer and stripe with just the bit stream. As a consequence, random access of rectangular areas and the like in an image is impossible, and the bit stream must be decoded from the beginning.